The present invention relates to novel X-ray or gamma-ray systems capable of detecting and resolving X-rays or gamma-rays of different energy levels.
X-ray and gamma-ray systems have been widely used over the last several decades in the industry and in medicine. Conventional radiography systems include an X-ray source (or a gamma-ray source) for producing a beam of X-rays (or gamma-rays) transmitted through an object, and an X-ray detector (or a gamma-ray detector) for detecting the transmitted radiation on a film or electronically. Alternatively, an X-ray detector may be located relative to the X-ray source to detect scattered X-rays. Electronic X-ray detectors directly convert X-rays to electrical signals, or indirectly detect X-rays by first converting them to secondary optical radiation and then detecting the secondary optical radiation. The electrical signals are then digitized and processed. Digital radiography provides numerous advantages over conventional radiography. The digital data is immediately available and can be processed and enhanced depending on the information to be extrapolated. Furthermore, the digital data may provide quantitative attenuation of the object when the data is calibrated and interpreted in absolute units.
An X-ray source can emit a broad spectrum of X-rays generated by a high-energy electron beam striking a metallic target. X-rays emitted from such polychromatic source are collimated to form a stationary or scanning pencil beam, a fan beam, or a wide area beam. Alternatively, an X-ray source can emit X-rays having two or several energy bands, wherein each energy band of X-rays may be quite narrow and thus considered mono-energetic.
Attenuation of X-rays depends on the thickness of the examined object, its density and its components. Different elements exhibit different relative attenuation of X-rays depending on their atomic number (Z). In the range of about 30 keV to 150 keV, the X-ray attenuation is characterized by three effects: Compton scattering, photoelectric absorption, and coherent scattering. Each material can be identified (i.e., atomic number Z can be calculated) based on its X-ray attenuation arising from Compton scattering and photoelectric absorption. Specifically, Compton scattering is dominant for the low atomic number materials and relatively small for high atomic number materials. Photoelectric absorption is more prevalent for high atomic number materials. Therefore, some medical or industrial applications may need multiple X-ray energy data.
X-ray imaging has been widely used for industrial testing and detection. A conventional X-ray image records the transmission of an object to a broad spectrum of X-rays. An X-ray inspection device uses an X-ray source emitting X-rays collimated relative to an X-ray detector, and a processor for analyzing the detected data and creating an image. X-rays examine or scan an object by a relative movement of the X-ray beam and the object. An X-ray inspection device may use line scanning for inspecting moving objects. Such inspection device uses a conveyor for moving an object, an X-ray source emitting a fan beam of X-rays directed toward an X-ray line detector. An object, usually moving at a constant speed on a conveyor, crosses the fan beam and thus causes X-ray flux variation at the detector. The detector provides successive lines of X-ray data used to create an image.
X-ray imaging has also been widely used in medicine. X-ray based systems are widely used for imaging and for bone densitometry. A conventional X-ray system transmits a wide-area X-ray beam through a patient and the transmitted x-rays are detected by a film or by a detector plate to obtain an image (for example, a chest X-ray). Other medical systems may collimate the generated X-rays into a pencil beam or a fan beam to scan a patient. An X-ray bone densitometer can use a scanning pencil beam or a fan beam directed to a point detector or a linear array of detectors, respectively. The detected radiation depends on the bone density, which is measured to evaluate and/or monitor osteoporosis.
In a CT system a thin fan beam of X-rays is directed through a patient""s body and detected by a line detector, usually including a line of individual sensors aligned along an arcuate or linear path. The examined patient is movably interposed between the source and the detector, which are always aligned with respect to the X-ray beam. For each position of the examined patient, the detector detects a fan beam of transmitted X-rays and produces a row of analog signals. The analog signals are digitized and provided to a processing unit that processes the data and provides an image to a display.
Dual energy radiography has been successfully used for medical applications and for detection and characterization of materials. A dual-energy X-ray system uses a single energy, a dual-energy or a multiple-energy X-ray source and at least two energy discriminant detectors. (Alternatively, a dual-energy X-ray system may use a source providing dual energy X-ray pulses and a single detector.) A first and a second energy detector may include a scintillating layer or material (e.g., a scintillating crystal) and optical detector (e.g., a photo diode or a phototransistor) located adjacent to the scintillating material. The two X-ray sensitive layers are aligned serially with respect to the direction of the X-rays. Low energy X-rays are more absorbed in the first sensitive area, while high energy X-rays are more absorbed in the subsequent X-ray sensitive layer. Thus, the first detector is frequently called a xe2x80x9cthinxe2x80x9d detector and the second detector is called a xe2x80x9cthickxe2x80x9d detector due to its ability to detect higher energy (harder) X-ray radiation. The individual detectors can be calibrated using two reference materials. The first reference material is usually a plastic such as Plexiglass(trademark) and the second is aluminum. The detector detects high energy and low energy values for known material thickness. Only two basic materials are required to determine the thickness-signal relationship for any other material.
Dual-energy X-ray detectors detect separately X-rays penetrating to different depth ranges based on their energies. Such X-ray detector includes a first strip of crystalline amorphous silicon (or another X-ray sensitive material) having a first surface in which X-rays of a first energy are absorbed. The first surface is disposed substantially perpendicularly to the incident X-rays. A second strip of crystalline or amorphous silicon has a surface in which X-rays of a second energy are absorbed. The second surface is again disposed substantially perpendicularly to the incident X-rays. The X-ray detector may also include an absorber (for example, an X-ray absorbing foil) stacked upstream from the first detector surface. The two X-ray detectors are stacked perpendicularly relative to the direction of X-rays and detect discrete X-ray energies (or energy bands), but cannot detect a continuum of X-ray energies.
Recently, new types of X-ray detectors have been using wide-area X-ray sensitive layers for direct or indirect detection of X-rays. A typical detector may include a two-dimensional, X-ray sensitive area with a plurality of individual sensors oriented substantially perpendicularly to incoming X-rays. These sensors provide again analog signals that are digitized and provided to a processing unit. However, these X-ray detectors cannot discriminate a spectrum or a continuum of X-ray energies.
Two-dimensional X-ray detectors may in the future replace X-ray films. A two-dimensional X-ray detector includes a two-dimensional array of sensors connected to a switching and addressing circuitry. The individual sensors typically include a pair of generally co-planar conductive microplates separated by a dielectric layer forming a charge storage capacitor. A photoconductive layer extends over all the sensors above the microplates. The X-ray detector also includes a top electrode placed over the photoconductive layer. A charging voltage is applied between the bottom microplates of all sensors and the top electrode to create an electric field across the photoconductive layer. The detected X-ray radiation irradiates the area of X-ray sensitive layer, i.e., the two-dimensional array is oriented perpendicularly relative to the direction of the X-rays. The received X-rays create electron-hole pairs, which under the influence of the applied electric field migrate across the photoconductive layer. The migration results in the accumulation of charge on the charge storage capacitors formed by the two microplates. The amount of charge stored in the storage capacitors varies proportionally to the radiation exposure. The stored charge is read-out and processed to create an image.
In the above identified applications, there still is a need to detect different energy levels of X-rays (or gamma-rays). There is a need for reliable energy sensitive detection systems capable of detecting discrete or continuous energy levels of X-rays and gamma-rays.
The present invention is directed to novel X-ray and gamma-ray systems and methods for medical, industrial or other applications. These systems use energy sensitive X-ray (or gamma-ray) detection systems capable of detecting and resolving X-rays (or gamma-rays) of different energy levels. Furthermore, the X-ray (or gamma-ray) detection system can directly convert X-rays (or gamma-rays) to electrical signals. Alternatively, the present X-ray (or gamma-ray) detection system can use a scintillating material to convert X-rays (or gamma-rays) to optical radiation that is detected and converted to electrical signals. Importantly, the detection system can discriminate different X-ray (or gamma-ray) energy levels.
According to one aspect, an X-ray detection system includes a photoconductive material, at least two electrodes, an X-ray shield, and a read-out circuitry. The photoconductive material and the electrodes are co-operatively arranged to provide an electric field inside the material in a first (field) direction. The X-ray shield, constructed and arranged to absorb X-ray radiation, includes an X-ray window arranged to direct X-rays into the photoconductive material in a selected X-ray direction. The X-ray direction may be oriented at an angle in the range of about 5xc2x0 to 90xc2x0 relative to the field direction, and preferably in the range of about 45xc2x0 to 90xc2x0 relative to the field direction, and more preferably 90xc2x0 relative to the field direction. The read-out circuitry is arranged to receive electric signals corresponding to the absorption of X-ray radiation at different depths in the photoconductive material.
According to another aspect, an X-ray detection system includes at least two electrodes constructed and cooperatively arranged at a photoconductive material to provide an electric field inside the material in a first direction, an X-ray shield and an X-ray window, a read-out circuitry, and a processing circuitry. The X-ray shield and the X-ray window are constructed and arranged to direct X-rays into the photoconductive material in a selected X-ray direction substantially different from the first direction, wherein the directed X-rays are absorbed at a location inside the photoconductive material at a depth corresponding to their energy. The read-out circuitry is constructed and arranged to receive electric signals corresponding to the absorption location inside the photoconductive material. The processing circuitry is constructed and arranged to determine the energy of the absorbed X-rays based on the electric signals.
According to yet another aspect, an X-ray system includes an X-ray source and an X-ray detection system. The X-ray source is constructed and arranged to emit X-rays having at least two different energies. The X-ray detection system includes at least two electrodes constructed and cooperatively arranged at a photoconductive material to provide an electric field inside the material in a first direction, an X-ray shield and an X-ray window, a read-out circuitry, and a processing circuitry. The X-ray shield and the X-ray window are constructed and arranged to direct X-rays into the photoconductive material in a selected X-ray direction substantially different from the first direction, wherein the directed X-rays are absorbed at a location inside the photoconductive material at a depth corresponding to their energy. The read-out circuitry is constructed and arranged to receive electric signals corresponding to the absorption location inside the photoconductive material. The processing circuitry is constructed and arranged to determine the energy of the absorbed X-rays based on the electric signals.
Preferred embodiments of these aspects include one or more of the following features:
At least one of the electrodes belongs to electron optics constructed and arranged to deflect an electron beam over a surface of the photoconductive material. The electron optics is co-operatively operates with the read-out circuitry to register the depth of the location when providing the electric signals. The read-out circuitry is arranged to integrate the excited charges over time.
The electrodes are arranged to apply a voltage directly to the photoconductive material to create the electric field inside the photoconductive material in the first direction. The electrodes include a first high-voltage electrode disposed on a first side of the photoconductive material and an array of electrodes distributed over a second side of the photoconductive material. The read-out circuitry is connectable to each electrode of the electrode array. The read-out circuitry is connectable to each the electrode of the array and is arranged to integrate the excited charges over time.
The photoconductive material is a film of a selected thickness or a plate. The photoconductive material is a film of a selected thickness; the electrodes are arranged to apply a voltage directly to the photoconductive material across the thickness to create the electric field inside the photoconductive material in the first direction; and the X-ray shield and the X-ray window are arranged to direct the X-rays to an edge of the film. The X-ray shield and the X-ray window are arranged to direct the X-rays to the edge having the X-ray direction being substantially perpendicular to the first direction. Alternatively, the X-ray shield and the X-ray window are arranged to direct the X-rays into the film, wherein the selected X-ray direction is oriented at an angle of about 45xc2x0 to 90xc2x0 relative to the first direction.
According to another aspect, a method of detecting X-ray radiation includes providing a photoconductive material cooperatively arranged with at least two electrodes, providing an electric field inside the photoconductive in a first direction, and shielding X-ray radiation for directing X-rays into the photoconductive material in a selected X-ray direction substantially different from the first direction. The directed X-rays are absorbed at a location inside the photoconductive material having a depth corresponding to their energy. The method further includes detecting charges excited at the absorption location inside the photoconductive material, and reading out electric signal corresponding to the excited charges.
According to yet another aspect, an X-ray sensitive apparatus includes a photo-conductive plate having a generally planar surface and being disposed inside of a scanning housing that also provides shielding. An electron beam scans the surface of the photo-conductive plate within the housing that includes a X-ray window for receiving of X-rays emitted from an external X-ray source. The housing and the X-ray window are arranged to collimate X-rays as to contact the photo-conductive plate at a selected acute or obtuse angle to the scanned surface.
Preferably, the external X-rays enter the housing through the window oriented perpendicularly to a side surface of the photo-conductive plate and in parallel to the scanned surface. The apparatus further includes an image processor constructed and arranged to receive a current generated in the photo-conductive plate due to the absorption of X-rays at different depths of the photo-conductive plate. The apparatus further includes a processor using an algorithm for generating color data or pseudo color data from current signals corresponding to the absorption of X-rays at different depths.
According to yet another aspect, an apparatus for sensing X-rays includes a photoconductive plate having a depth selected to receive, from an external X-ray source, X-rays having a selected distribution of energies which penetrate the photoconductive plate along its depth. The plate includes an array of electrodes arranged along the depth of the photoconductive plate in a pattern selected to sense an electrical signal created by the absorption of X-rays within the photoconductive plate.